This invention relates generally to gas turbine engines and more particularly to the repair of turbine nozzle segments used in such engines.
A gas turbine engine includes a compressor that provides pressurized air to a combustor wherein the air is mixed with fuel and ignited for generating hot combustion gases. These gases flow downstream to a turbine section that extracts energy therefrom to power the compressor and provide useful work such as powering an aircraft in flight. Aircraft engines typically include stationary turbine airfoils, herein referred to as vanes, that enhance engine performance by appropriately influencing gas flow and pressure within the turbine section. In multi-stage turbine sections, turbine vanes are placed at the entrance of each turbine stage to channel combustion gases into the rotating turbine blades located downstream of the vanes. Turbine vanes are typically segmented around the circumference thereof with each vane segment, herein referred to as a nozzle segment, having one or more vanes disposed between inner and outer bands that define the radial flowpath boundaries for the hot combustion gases flowing through the nozzle segment. These nozzle segments are mounted to the engine casing to form an annular array with the vanes extending radially between the rotating blades of adjacent turbine stages.
Nozzle segments are located directly in the gas stream and therefore are exposed during operation to high temperature, corrosive air that can limit the effective service life of these components. Accordingly, nozzle segments are typically fabricated from high temperature cobalt or nickel-based superalloys and are often coated with corrosion and/or heat resistant materials. Furthermore, nozzle segments are ordinarily cooled internally with cooling air extracted from the compressor to prolong service life. Even with such efforts, portions of the nozzle segments, particularly the vanes, can exhibit cracks, corrosion, and other damage such that the nozzle segments must be either repaired or replaced to maintain safe, efficient engine operation. Because nozzle segments are complex in design, are made of relatively expensive materials, and are expensive to manufacture, it is generally more desirable to repair them whenever possible.
One existing repair technique is described in U.S. Pat. No. 6,416,278, issued to the assignee of the present invention. This technique involves salvaging an inner band and attaching it to a newly manufactured replacement inner band segment which includes an outer band and one or more vanes. This is possible because the inner band typically experiences less severe operating conditions and has a longer life than the other nozzle segment structure. However, the repair process itself incrementally reduces the life of the inner band because the processes used to clean and strip old protective coatings also removes base material of the inner band further reducing the structural capability of the part. At a subsequent repair cycle, this may result in certain portions of the inner band structure, herein referred to as an inner band segment, having a wall thickness that is less than an acceptable minimum for re-use, while the previously replaced outer band and vanes are nearly new.
Accordingly, to avoid scrapping the entire nozzle segment in such a situation, it would be desirable to have a method for salvaging the previously replaced and repairable portion of the nozzle segment.